A computing environment comprising, for example, a CPU, memory and Input/Output (I/O) devices, typically includes an operating system to provide a way to control the allocation of the resources of the environment. Traditional multitasking operating systems (e.g., UNIX, Windows) have been implemented in computing environments to provide a way to allocate the resources of the computing environment among various user programs or applications that may be running simultaneously in the computing environment. The operating system itself comprises a number of functions (executable code) and data structures that may be used to implement the resource allocation services of the operating system.
Certain operating systems, called “real-time operating systems,” have been developed to provide a more controlled environment for the execution of application programs. Real-time operating systems are designed to be “deterministic” in their behavior—i.e., responses to events can be expected to occur within a known time of the occurrence of the event, without fail. Determinism is particularly necessary in “mission-critical” and “safety-critical” applications, where the outcome of event responses is essential to proper system function. Real-time operating systems are therefore implemented to execute as efficiently as possible with a minimum of overhead. As a result, prior real-time operating systems have typically employed relatively simplistic protection models for system and user processes—typically all processes execute in the same space, thus allowing direct access to all system resources by all user tasks (system calls can be made directly). This real time operating system model provides the fastest execution speed, but is deficient in providing system protection.
In order to improve system protection, it has been proposed to provide an operating system that implements a “protection domain” architecture. VxWorks® AE, marketed by Wind River Systems of Alameda, Calif., is an example of such a protection domain system. Basically, the protection domain system segregates the computing environment into a number of “protection domains.” Each protection domain is a “container” for system resources, executable code and data structures, as well as for executing tasks and system objects (such as semaphores and message queues). Each resource and object in the system is “owned” by exactly one protection domain. The protection domain itself is a self-contained entity, and may be isolated from other system resources and objects to prevent tasks executing in the protection domain from potentially interfering with resources and objects owned by other protection domains (and vice versa).
The protection domain system of VxWorks® AE also, however, provides mechanisms by which tasks executing in one protection domain may access resources and objects contained in a separate protection domain. Each protection domain includes a “protection view” that defines the system resources and objects to which it has access (i.e., the resources and objects which it can “see”). By default, each protection domain has a protection view that includes only the system resources and objects contained within that protection domain. However, a protection domain may acquire access to the resources of other protection domains by “attaching” to these protection domains. When a first protection domain has obtained “unprotected attachment” to a second protection domain, the second protection domain is added to the protection view of the first protection domain. Executable code in the first protection domain may use “unprotected links” to functions selected in the second protection domain, allowing tasks executing in the first protection domain to use the resources and access the objects of the second protection domain with a minimum of execution overhead.
Unrestricted access by all tasks executing in one protection domain to all the resources and objects of another protection domain may not be desirable, however, for reasons of system protection and security. The VxWorks® AE protection domain system therefore provides a further mechanism whereby individual tasks executing in a first protection domain may access resources or objects contained in a second protection domain, but without adding the second protection domain to the protection view of the first protection domain. This access is achieved by “protected attachment” of the first protection domain to the second protection domain via a “protected link” between executable code in the first protection domain and selected functions in the second protection domain. Using the protected link, a task running in the first protection domain may, for example, make a direct function call to a function existing in the second protection domain, without the need to alter the protection view of the first protection domain. Tasks in the first protection domain are prevented from accessing the second protection domain except through this protected link, thus preventing unauthorized accesses of functions and data in the second protection domain. Protected linking can be achieved without the need to use different code instructions for protected and unprotected accesses (increasing implementation flexibility), and without the need to create separate tasks in the protected protection domain to perform the desired actions.
Such a protection domain system allows the operating system to dynamically allocate system resources among processes and flexibly implements and enforces a protection scheme. This protection scheme can be formulated to control the impact of poorly written applications, erroneous or disruptive application behavior, or other malfunctioning code, on the operating system and other applications running in the computer system. The protection domain approach accomplishes the protection results in a manner that is transparent to application developers, and incurs minimal execution overhead.